The ultrasonic propagation properties of living system, and preparations of their molecular constituents, are studied with the view toward obtaining a more profound understanding of the fate of the wave process in such media and toward providing information to insure effective and safe clinical use of this modality. Special attention is devoted to absorption properties of tissues and the physical mechanisms responsible for loss of energy. The role and influence of degree of structure on the absorption of ultrasound by proteins (in solution) is emphasized. Recent findings suggest that mammalian gonads have relatively low absorption coefficients and that structural proteins may provide for greater ultrasonic attenuation than do the globular proteins. Development of measurement methods and instrumentation is also include in the program.